<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body
        {
            font-size: 60px;

        }
        .box1{
            width: 200px;
            height: 200px;
            background-color: #bfa;
            /* 开启了定位元素：可以通过z-index属性来指出元素的层级
             z-index需要一个整数作为参数，值越大元素层级越高
               元素的层级越高越越优先显示
            */
            /* 祖先元素的层级再高也不会盖住后代元素 */
            z-index: 1;
        }
        .box2{
            width: 200px;
            height: 200px;
            background-color: orange;
            position: absolute;
            top: 100px;
            z-index: 2;
        }
        .box3{
            width: 200px;
            height: 200px;
            background-color: #faa;
            position: absolute;
            z-index: 3;
        }
        .box4{
            width: 100px;
            height: 100px;
            position: absolute;
        }
    </style>
</head>
<body>
    <div class="box1">1</div>
    <div class="box2">2</div>
    <div class="box3">3
        <div class="box4">4</div>
    </div>
</body>
</html>